In many applications a power converter is required to provide an output voltage within a predetermined range formed from an output voltage source having a different voltage level. One very common type of power converter is a flyback type voltage converter.
FIG. 1 illustrates a conventional flyback type voltage converter. The converter 2 includes a transistor Q1, a controller 4, a transformer TX, a capacitor C1, and a diode D1. Input voltage to the circuit may be unregulated DC voltage derived from an AC supply after rectification and filtering. The transistor Q1 is a fast-switching device, such as a MOSFET, the switching of which is controlled by a fast dynamic controller 4 to maintain a desired output voltage Vout. The secondary winding voltage is rectified and filtered using the diode D1 and the capacitor C1. The transformer TX of the flyback converter functions differently than a typical transformer. Under load, the primary and secondary windings of a typical transformer conduct simultaneously. However, in the flyback converter, the primary and secondary windings of the transformer do not carry current simultaneously. In operation, when the transistor Q1 is turned ON, the primary winding of the transformer TX is connected to the input supply voltage such that the input supply voltage appears across the primary winding P1, resulting in an increase of magnetic flux in the transformer TX and the primary winding current Ipri rises linearly. However, with the transistor Q1 turned ON, the diode D1 is reverse biased and there is no secondary current Isec through the secondary winding S1. Even though the secondary winding S1 does not conduct current while the transistor Q1 is turned ON, the load, represented as resistor Rload, coupled to the capacitor C1 receives uninterrupted current due to previously stored charge on the capacitor C1.
When the transistor Q1 is turned OFF, the primary winding current path is broken and the voltage polarities across the primary and secondary windings reverse, making the diode D1 forward biased. As such, the primary winding current is interrupted but the secondary winding S1 begins conducting current thereby transferring energy from the magnetic field of the transformer TX to the output of the converter. This energy transfer includes charging the capacitor C1 and delivering energy to the load. If the OFF period of the transistor Q1 is sufficiently long, the secondary current Isec has sufficient time to decay to zero and the magnetic field energy stored in the transformer TX is completely dissipated.
The flyback topology has long been attractive because of its relative simplicity when compared with other topologies used in low power applications. A drawback of the flyback type converter, as with all power converters, is power loss incurred within the circuit. As such, a general goal in power converter circuit design is to minimize power loss. A primary source of power loss in the flyback type converter is the secondary side diode D1 because the diode D1 carries the entire secondary side current Isec. This source of power loss is not limited to flyback type converters, but to any power converter having a secondary side diode that carries secondary side current. By way of example, a conventional application for a flyback type converter is to convert AC wall voltage to a DC voltage for low power applications, such as 5V to charge a cellular telephone. In this case, the secondary side current is approximately 2 A resulting in output power of 10 W. For a conventional diode having a forward voltage of 0.7V, this results in power dissipation across the diode of 1.4 W, or 14% of the total power output.
Synchronous rectification is a technique for improving the efficiency of rectification by replacing diodes with actively controlled switches such as transistors, typically power MOSFETs or power BJTs. In the case of power converters, such as the flyback type converter of FIG. 1, a synchronous rectifier is implemented as a MOSFET that replaces the diode D1. FIG. 2 illustrates a conventional flyback type voltage converter implemented using synchronous rectification. The flyback type converter 4 of FIG. 2 is similar to the flyback type converter 2 of FIG. 1 except that the diode D1 of converter 2 is replaced by a synchronous rectifier, a transistor Q2. The diode D2 represents the intrinsic diode, also referred to as the parasitic or body diode, of the transistor Q2. The synchronous rectifier driver circuit first allows the body diode D2 to conduct then the transistor Q2 is turned ON. The synchronous rectifier is more efficient than a diode for conducting the secondary side current Isec. By way of example, the transistor Q2 implemented as a MOSFET has an on resistance Rdson of approximately 0.04 ohms. In the case of the 2 A secondary side current, the power dissipation across the transistor Q2 is (Isec)2(Rdson), which is approximately 0.16 W, significantly lower than the 1.4 W power dissipation across the diode D1 in FIG. 1.
An issue associated with using the synchronous rectifier is how to efficiently and economically turn ON the transistor Q2. The use of a synchronous rectifier driver circuit employs an additional secondary side winding that couples a primary side controller signal to synchronize the timing with the synchronous rectifier. FIG. 3 illustrates a conventional flyback type voltage converter implemented using synchronous rectification and a synchronous rectifier driver circuit. The flyback type converter 8 of FIG. 3 is similar to the flyback type converter 6 of FIG. 2 with the addition of a synchronous rectifier driver circuit. The synchronous rectifier driver circuit includes additional secondary winding S2, a diode D3 and a capacitor C2 configured to supply sufficient gate-to-source voltage Vgs to turn ON the transistor Q2 at an appropriate time during the power delivery cycle. The synchronous rectifier driver circuit is self driven to automatically drive the transistor Q2. The additional secondary winding S2 ensures that the secondary side control is in step with the primary side control. However, introduction of the additional secondary winding S2 increases the complexity of the transformer design.
A fundamental issue with a MOSFET is that it is not strictly a uni-directional current element when turned ON. Positive secondary current Isec or negative secondary current Isec can pass through the transistor Q2 when the transistor Q2 is turned ON depending on the voltage polarities. As such, the transistor Q2 can not simply be turned ON and left ON. When the primary side transistor Q1 is turned OFF, there is positive voltage across the secondary windings S1 and S2, which results in the transistor Q2 turning ON and positive secondary current Isec flowing from the secondary winding S1 through the transistor Q2 to the output capacitor C1 and the load Rload. However, when the primary side transistor Q1 is turned ON, there is negative voltage across the secondary windings S1 and S2. Negative voltage is not applied to the gate of transistor Q2 and the transistor is left “flying”, or ON. The positive voltage across the output capacitor C1, which is supposed to provide current flow from the capacitor C1 to Rload during this period, instead flows through the turned ON transistor Q2 as negative secondary current Isec. In other words, the capacitor C1 discharges through the secondary winding S1 instead of the load. So, in this case, when turned ON the transistor Q2 functions as a bi-directional switch.
An approach for appropriately turning OFF the synchronous rectifier is to employ current detection to maintain an ON-state of the synchronous rectifier until the secondary current Isec reaches zero. Zero secondary current indicates a dead time period which is a signal to turn OFF the synchronous rectifier until the next cycle. FIG. 4 illustrates another conventional flyback type voltage converter implemented using synchronous rectification and a synchronous rectifier driver circuit. The flyback type converter 9 of FIG. 4 is similar to the flyback type converter 6 of FIG. 2 with the addition of a synchronous rectifier driver circuit. The synchronous rectifier driver circuit 9 uses current detection and includes a resistor R1 added into the secondary current path. A comparator 7 measures the voltage across the resistor R1 against a reference value and outputs the difference to a controller 5. When the controller 5 determines zero voltage across the resistor R1, which corresponds to zero secondary current, an output of the controller 5 triggers a turn OFF signal to the synchronous rectifier Q3.
Other techniques that use current detection configure an additional secondary winding in series with the synchronous rectifier, in contrast to the parallel configuration shown in FIG. 3. A series configuration reduces circuit complexity but does not completely alleviate the increased complexity of the transformer due to the additional secondary winding.
Another approach for appropriately turning OFF the synchronous rectifier is to employ voltage detection across the drain-to-source of the synchronous rectifier. The drain-to-source voltage Vds can be detected using sensing circuit that includes a differential amplifier coupled to the drain and the source of the synchronous rectifier. When the sensing circuit detects that the voltage Vds is low, the sensing circuit sends a signal to a driving circuit to turn ON the synchronous rectifier until a reverse biased condition is determined in the differential amplifier, at which point the driving circuit is signaled to turn OFF the synchronous rectifier. Although effective, such a configuration needs current build up to drive the synchronous rectifier, which results in a delay during turn ON. Each of the conventional approaches that use either current or voltage detection also use the additional secondary winding.
Additional circuit complexity is also needed to properly time turning ON the synchronous rectifier. Care must be taken to avoid turning ON the intrinsic diode, such as the diode D2 in FIG. 3, before the synchronous rectifier turns ON. The intrinsic diode has a similar power loss as a stand-alone diode. If the intrinsic diode turns ON and conducts, the purpose of using the synchronous rectifier instead of a stand-alone diode is defeated. Such timing circuitry is typically included between any sensing circuitry and the synchronous rectifier.